Antenna Calibration (AC) plays an important role in an RF transceiver. For example, beamforming performance of an RF transceiver is dependent on the AC accuracy.
For beamforming, it is generally required that radio branches in an RF transceiver are convergent, i.e., having the same phase and magnitude responses. Hence, each radio branch should be calibrated against the other branches in terms of phase and magnitude responses. However, the phase response is likely to differ between radio branches as they may have different feeder lengths and different internal analog filters.
The AC technique has been widely adopted in multi-antenna radio systems to enhance their beamforming performances. A typical AC includes measuring and calculating relative transfer functions between radio branches, calculating compensation coefficients and applying the compensation coefficients to compensate for differences in phase and magnitude responses between the radio braches.
Conventionally, in order to obtain the phase and magnitude responses of a radio branch connected with an antenna, an external coupler is provided very close to the antenna or built-in to the antenna. This is so-called external AC. Alternatively, an internal AC technique has been proposed. For most of site solutions, the antenna is very close to the radio unit, the difference in feeder lengths between radio branches could be negligible. So, an internal coupler unit can be deployed inside the radio unit. The internal AC is an important aspect to fulfill the AC function without any auxiliary hardware outside the radio unit. The basic idea of the internal AC is to provide a measurement transmitter and a measurement receiver and to compare the differences in phase and magnitude responses between the radio braches using internal Voltage Standing Wave Ratio (VSWR) Forward (FWD) couplers.
FIGS. 1A and 1B are schematic diagrams showing an RF transceiver 100 with internal AC. As shown in FIGS. 1A and 1B, the RF transceiver includes a radio unit 110 and a number of antennas 101, 102, 103 and 104. The radio unit 110 includes a number of radio branches each associated with one of the antennas, of which only one radio branch 111 is shown. The radio branch 111 includes a radio transmitter 121 and a radio receiver 122 for transmitting and receiving radio signals via the antenna 101. The radio unit 110 further includes a measurement transmitter 112 and a measurement receiver 113 for transmitting and receiving calibration signals, respectively. The radio unit 110 further includes a coupler unit 114 having a number of couplers (e.g., VSWR RWD couplers) each connected to one of the antennas. One of the couplers is shown at 123, which is also a part of the radio branch 111 and selectively connected to the radio transmitter 121 and the radio transmitter 122. The coupler unit 114 further includes a switch 124 for selectively connecting one of the measurement transmitter 112 and the measurement receiver 113 with one of the couplers.
FIG. 1A shows a signal flow for calibration associated with the radio receiver 122. As indicated by the arrows, a calibration signal is transmitted from the measurement transmitter 112 to the coupler 123 via the switch 124 and is received by the radio receiver 122 via coupling by the coupler 123. FIG. 1B shows a signal flow for calibration associated with the radio transmitter 121. As indicated by the arrows, a calibration signal is transmitted from the radio transmitter 121 to the coupler 123, coupled via the coupler 123 and is received by the measurement receiver 113 via the switch 124. The internal AC shown in FIGS. 1A and 1B can calibrate differences in phase and magnitude responses between the radio branches before the calibration plane as indicated by the vertical dashed line.
FIGS. 2A and 2B are schematic diagrams showing two types of AC sequences, serial AC sequence and parallel AC sequence, respectively. As shown in FIG. 2A, for the serial AC sequence, the radio branches are calibrated sequentially in different time slots. That is, when one radio branch is being calibrated, the other branches may have traffic or may be idle. As shown in FIG. 2B, for the parallel AC sequence, the radio branches are calibrated simultaneously in the same time slots.
However, interferences from Antenna Reference Point (ARP), i.e., external interferences entering an RF unit via an antenna, could adversely affect the AC accuracy, which in turn will degrade the beamforming performance.
FIGS. 3A and 3B show interferences in the AC scenarios of FIGS. 1A and 1B, respectively. In FIG. 3A, as indicated by the dashed line, the interference from ARP enters the radio receiver via the coupler. In FIG. 3B, as indicated by the dashed line, the interference from ARP enters the measurement receiver via the coupler and the switch. The AC accuracy is largely dependent on the Signal to Interference and Noise Ratio (SINR) at the radio receiver (FIG. 3A) or the measurement receiver (FIG. 3B). There are two parameters contributing to the SINR, the thermal noise, or Signal to Noise Ratio (SNR), and the interference, or Signal to Interference Ratio (SIR), i.e., SINR=SNR+SIR. Typically, due to limitations on characteristics of analog components, the thermal noise can only be restricted to a certain range. Then, the SIR is the only parameter which is critical to the optimization of the SINR.
For the calibration associated with the radio receiver as shown in FIG. 3A, the SIR can be improved by increasing the power level of the calibration signal transmitted by the measurement transmitter. However, the measurement transmitter (which operates at the same frequency as the radio receiver) generates spurious emission at the ARP, which should be limited to a specified power spectral density (e.g., lower than −85 dBm/MHz for Time Division Duplex (TDD) or −110 dBm/100 KHz for Frequency Division Duplex (FDD)). Such limitation results in a limited power level of the calibration signal.
For the calibration associated with the radio transmitter as shown in FIG. 3B, the measurement receiver suffers from strong in-channel or adjacent-channel interference, especially when there are other transceivers co-located. The SIR will be even worse if the transceiver is used in a low power station, e.g., a micro/pico base station or user equipment (UE). FIG. 4 shows a possible interference scenario where the transceiver is used in a base station. If the serial AC sequence is adopted, traffics in other radio branches could result in interferences through mutual antenna leakage (i.e., self-interference). That is, the AC has to suffer from cumulative interferences from all of other radio branches. In addition, there will be inter-station interferences from other base stations, which could be at maximum 25 dBm (considering a 50 dBm interference and a 25 dB antenna isolation).
There is thus a need for an AC solution with improved SIR and thus improved accuracy.